A preparation of toner for developing electrostatic images has been conventionally carried out in such a manner that: a coloring agent like dyestuff and pigment and optionally by a magnetic material, a charge controlling agent and an offset prohibitor are added to a molten thermoplastic resin, and mixed well and the mixture is cooled to solidify, followed by pulverization and classification.
The conventional method, however, has been confronting with troubles such as a multiplicity of steps and apparatuses, complex process and difficulty in producing toner of small grain sizes. Further it has a drawback that the produced powder has not only a wide grain distribution but also non-uniform shapes so that the material has a poor fluidity, resulting in fog of an image.
To prepare toner with a uniform grain shape, a small grain size and a narrower grain distribution, various polymerization processes have been proposed. For example, Japanese Patent Publication Nos. Sho 47-518305 and Sho 51-14895 disclose a method by a suspension polymerization, Japanese Patent Laid-Open No. Sho 63-282749 by an emulsion polymerization and Japanese Patent Laid-Open No. Hei 3-200976 by a dispersion polymerization.
However, the toners produced by such polymerization methods irresistibly contain a dispersing agent which is used in the polymerization process. Thus the toners prepared by polymerization methods are in general sensitive to humidity, which is likely to deteriorate charge properties and fluidity. Moreover, there still exist problems that; kinds and amounts of applicable charge controlling agent are so restricted as not to prevent polymerization reaction; electrified quantities may fluctuate among the toner particles due to inhomogeneous dispersion state of the charge controlling agent; and the charge controlling agent is present within the toner particles, which do not contribute to friction electrification, leading to an insufficient utilization of the charge controlling agent.
Various attempts have been done to improve the charge properties of toner produced by polymerization methods. For example, Japanese Patent Laid-Open Nos. Sho 62-226162 and Sho 64-59239 disclose a toner preparation method, wherein organic resin fine particles are adhered to surfaces of colored polymer particles. But in adhesion of organic resin fine particles to the surfaces of the colored polymer particles through an electrostatic interaction, the surface charges of both organic resin fine particles and colored polymer particles must be controlled so that kinds of applicable materials will be limited and amounts of the organic resin fine particles to be adhered will be insufficient. Also in the method wherein organic resin fine particles and colored polymer particles are dried separately and then the surface adherence is conducted by mechanical blend, the cohesives are apt to be yielded at the time of particle drying so that it is difficult to adhere to the surfaces in the form of primary particles.
Japanese Patent Laid-Open Nos. Hei 4-67155 and Hei 4-213464 disclose a preparation method wherein a charge controlling agent is adhered or settled to the surfaces of colored polymer particles through an insolubilization, etc. This method is effective in utilizing the charge controlling agent but on the other hand the adherence of the conditioner is insufficient and as a result the charge controlling agent removes readily from the surfaces of the colored polymer particles leading to a charging hindrance in actual service.
There is also described a method in Japanese Patent Laid-Open No. Hei 1-93748 that agglomerates of organic resin fine particles are formed by salting-out and after drying, coloring agents etc. are adhered thereon by applying mechanical impact. This method, however, fails to assure charge stability.